The present invention relates to a method for the removal of nitrogen oxides, especially of NO and/or NO.sub.2 from waste gases of many kinds, especially waste gases obtained from industrial furnaces and from combustion firing systems. The present invention further relates to a method for the recovery of nitric acid.
For reasons of environmental protection, effective methods for an extensive removal of nitrogen oxides from industrial systems, e.g. the production of fertilizers or the processing of metals, but also from combustion furnaces systems of varied types, including power plants and garbage incineration plants, are becoming more and more important. The value of these methods is to be judged on the basis of the extent to which the pollutants can be removed and whether harmless conversion products are created in those methods or whether utilizable by-products, that is, valuable substances, are formed.
Methods have already been described by which waste gases of varied origins containing nitrogen oxide were subjected to a gas scrubbing operation for the physical or chemical absorption of the nitrogen oxides. Since primarily nitrogen dioxide, NO.sub.2, and nitrogen monoxide, NO, are contained in such waste gases and since the latter is especially difficult to absorb in aqueous media, the suggestion has been made in DE-AS 25 37 061 that the NO be catalytically oxidized before the gas scrubbing.
The nitrogen dioxide formed can be removed from the waste gas basically by means of absorption in water. However, a sufficient reduction of its concentration can not be achieved in a satisfactory manner in the case of low concentration waste gases (e.g. &lt;2000 ppm NO.sub.2) when using the conventional apparatus in practice in large-scale purification of waste gases. This is due to the economically imposed conditions limiting the dwell times (e.g. to values below approximately 2 sec.) as a consequence of the rate of dissolution, which is still too low.
Even a reaction of dissolved nitrogen dioxide by means of a chemical reaction with oxidation agents, alkali metal hydroxide or ammonia, which are added to the absorption agent with the intention of achieving an acceleration of the absorption by means of a spontaneous removal of dissolved nitrogen dioxide from the solution equilibrium, has not resulted in the desired success. In spite of the addition of NaClO.sub.2 as an oxidation agent, rates of absorption which were barely greater than 50% were able to be achieved under industrial conditions, that is, in the case of NO.sub.x amounts of Y.sub.NOx .ltoreq.1000 ppm, dwell times in a waste-gas scrubber of &lt;1 sec. and temperatures &gt;40.degree. C. An aggravating circumstance is the fact that, at least in the case of combustion waste gases, alkaline scrubbing solutions can not be economically used because of the alkali consumption by CO.sub.2. Even the addition of solubilizers (metal-chelate complexes) was not able to improve oxidative gas scrubbing to any considerable degree.
The currently dominant method for the denitration of waste gases relies on the reduction of nitrogen oxides to nitrogen and water. This can take place either catalytically (SCR) or non catalytically by means of the use of high temperatures (SNCR). Ammonia is usually used as the reduction agent in both methods. However, SCR and SNCR systems for the denitration of waste gases have the disadvantage of a greater or lesser ammonia leakage.
There are other disadvantages in addition to the ammonia leakage. There is the necessity of keeping in reserve rather large amounts of liquid ammonia, which is associated with a potential danger for the surrounding population. The SCR method requires relatively expensive catalysts. Since both methods are carried out at an elevated temperature, the accumulating waste gases, which are cooler, must be preheated. The harmless compounds nitrogen and water are produced as reaction products; they do not constitute useful substances which can be used further.
An attempt has also been made to treat nitrogen-oxide-containing waste gases with aqueous solutions of hydrogen peroxide in a scrubbing method. This method has also not resulted in the past in any palpable success since the degrees of denitration achievable in the case of waste gases containing a low concentration of nitrogen oxides and requiring rather long scrubbing times were not even approximately sufficient.
An object of the invention is to provide a method for the purification of nitrogen-oxide containing waste gases, especially those with low concentrations of NO.sub.x, that is, concentrations &lt;1000 ppm NO.sub.x, which permits degrees of denitration above 90% even at low temperatures and drastically reduced dwell times and in which the nitrogen oxides can be converted practically completely into a useful substance, namely, to nitric acid.